reactive decay modes of carbonyl excited states · carbonyl triplet addition to electron poor...
TRANSCRIPT
13.1
Reactive decay modesof carbonyl excited states
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts
13.2
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts
k rxn
The transformation may involve atom or electron transfer or bond breaking or formation. It is usually an adiabatic process. Spin is conserved
A small number of elementarytransformations are normallysufficient to explain most photo-reactions, which may reflectcombinations of this basic reaction set
Reactive decay modesof carbonyl excited states
13.3
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts
Ground state species, in principle stable butwhose lifetime is limitedby intramolecular orintermolecular reactivity
The nascent reaction intermediates usually remember the spin state of the precursor excited state
Typical examples
Free radicalsbiradicalsradical ionscarbocationsylidesortho-xylylenescarbenes
13.4
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts
Transitionstate
‡
Reactiveexcited
state
Products
Intermediatespecies
13.5
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts
O
CH3
EXAMPLE
O*
CH3
hν k rxn • OH
CH2•
OOH
CH2CH2
O
CH 3
OH
Ph
Product formingreactions
13.6
Typical reactions of n,π* states
• Atom abstractions
• Radical-like addition
• Electron or charge transfer
• Bond cleavageas donor
as acceptor
α or β positions
IntermolecularIntramolecular
Electron rich and electron poor systems
13.7
O
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
Benzophenone triplet:a classic example
ISC
S1
T 1
S0
The benzophenone triplet has radical-like characteristics similar to those of an alkoxyl radical
13.8
Some hydrogen abstraction rate constants
(in units of 106 M
-1s-1 )
Benzophenonetriplet
(CH3)3CO•
Cyclohexane
Methanol
Benzhydrol
Triethylsilane
0.45
0.21
7.5
9.6
1.6
0.29
7.2
5.7
•Ph2 COH + R•Ph2CO* + R-H
Intermolecular hydrogen abstractions
13.9
The Norrish Type II reactionAn example of intramolecular hydrogen abstraction
H
O*
Ph
Me Me
OH
Ph
Me Me
OH
Ph
Me Me
OH
H
O
Ph
Me Me
Ph
n,π*triplet
100 ns
•
•2 ns
γMeVLP Acetophenone (ACP)hν
13.10
What determines rate constants for hydrogen abstraction?
• Nature of the excited state: n,π* states are radical-like and react faster
• Triplet energy ....... high is better
• Strength of the R-H (frequently C-H) bond broken
• Number of hydrogen atoms available
• Steric effects
• Polar effects ..... most excited carbonyl states are electrophillic
• In intramolecular examples, the ability to form an unstrained transition state ..... 6-center is good
• Entropic factors can be quite important in intramolecular cases
• .......... and the usual temperature, solvent, etc.
13.11
How are those rates determined?Indirect approach: The Stern-Volmer equation
γMeVLP Acetophenone (ACP)hν
hνγMeVLP γMeVLP*
γMeVLP* ACPk = τ-1
γMeVLP* + Q γMeVLP + Q*kq
Φisc
ΦACP
= Φisc
kk + kq[Q] Φ
ACP
0 = Φ
isc
with quencher no quencher
Φ
ACP
0
ΦACP
= 1 + kq[Q]
k = 1 + kq τ [Q]
13.12
Quenching of Benzophenone Triplets by Melatonin
0
1 106
2 106
3 106
4 106
5 106
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
k d
[melatonin], mM
0
0.01
0.02
0 1 2 3 4
∆OD
Time, µs
0.24 mM
k ~ 4 x 109
How are those rates determined?Direct approach: Time-resolved detection
NH
H3CO NH CH3
O
13.13
What determines efficiencies for hydrogen atom abstraction?
R2C=O* Unreactive decayτ–1
R2C=O* + R–H R2C–OH + R•kR
Φreaction = kR[RH]
τ−1 + kR[RH]
rate constant, concentration and lifetime
13.14
OCH3
CH3CH2
CH3 OH
hν
Two roads to the same enol
O H
H
O
Ph
Ph OH
Ph
H
Ph
OH
hν hν
Photoenolization:an example of a reversible hydrogen abstraction
13.15
Biradical or excited enol?
OHCH3
CH2
OHCH3
CH2
••
*
Radical reactions
Electron Transfer
Energy transfer
( β-carotene)
Oxygen sensitization
OHCH3
CH21 O2 +
luminescenceat 1.27 µm
Oxygen sensitization
13.16
Cyclobutanol formation
OCH 3CH 3 OH
CH3
CH3
CH3 CH3
H3C
H3C
HO CH 3CH3
CH3
hν
Biradical
13.17
Photorearrangement of o-chloro-α-methylacetophenone
CH2Cl
O
O
OH
CH2Cl
+ HCl
hν
13.18
Quenching of acetone fluorescence by Bu3SnH
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 t/ns
rela
tive
inte
nsity
O 1*
3.5 M0.90 M0.25 M0.0 M
350 400 450 500
rela
tive
in
ten
sity
in Me4Sn
in Bu3SnH
0.0
1 .0
0 .2
0 .4
0 .6
0 .8
wavelength, nm
O 1*
Bu3SnH vs. Bu4Sn
(singlet)
13.19
Reactivity versus Efficiency
Although more reactive , singlet-excited states are less efficient in abstracting hydrogen, i.e., lower yields of radicals result.
This result accounts for the observed low chemical reactivity of singlet-excited ketones
O3*
O
λmax = 390 nm
1*
+ H–SnBu320 %
O+ H–SnBu3
SnBu3
H
+
80 %
100 %
13.20
Experimental singlet and triplet reactivities
singlet triplet
tributyltinhydride 10 5.4
1,4-cyclohexadiene 2.1 0.63
isopropanol 0.091 0.010
O *
all rate constants in units of 108 M–1s–1
quencher
The higher energy of an excited SINGLET relative to the triplet state results in a HIGHER intermolecular REACTIVITY toward hydrogen donors.
13.21
Salem-type diagram
13.22
Norrish Type I reaction:an example of α-cleavage
CH3
CH3
O*
PhCH2
CH3
O
PhCH2
O*PhCH2
O
PhCH2 • + CO
~100 ns
•+ PhCH2 •
fast
slow+ CH3 •
•
A free radical analogue
CH3
CH3
O•
CH3 CH3
CH3
O + CH3•
13.23
Norrish Type I reactions in cyclic systems:processes in triphenylindanone
O
Ph
Ph Ph
O
Ph
Ph Ph
HPh
Ph
Ph
fast
fast ••
hν
•
•
Ph
Ph
H
Ph
Ph
Ph
PhPh
HPh
τ = 220 µ s
(-2H)
DPA TPBC
fast
Biradicals instead of free radicals
13.24
Electron transferA frequently masked reaction
•Ph2 CO• + Et3 N+• •Ph2 COH + CH3 CHN(C2H 5)2
Ph2CO* + (C2 H5 )3N Ph2 CO• + Et3 N+•
α-Aminoalkyl radicals: excellent reducing agents.
Ph2 COH + CH2 =CHN(C2H 5)2
•Ph2CO + CH3 CHN(C2H 5)2
•
13.25
Ketones as electron donors
7006005004003000.000
0.003
0.006
0.009
0.012
0.015
Wavelength/nm
OD
Ph2CH• + Ph2 CH+ + COPh2 CHCOCHPh2+•
Ph2CHCOCHPh2+•
acceptor
hνPh2 CHCOCHPh2
330 nm 440 nm
Tetraphenyl acetoneon an iron-rich clay
13.26
Transient spectrum of xanthone tripleta useful polarity probe
0
0.2
0.4
0.6
0.8
1
400 440 480 520 560 600 640 680 720
∆OD
Wavelength, nm
O
O
ethyl acetateethylene glycoltrifluorotoluene
13.27
Quenching of xanthone triplet with various aromatics
Benzene 7.6 x 105
Toluene 4.2 x 106
Pyridine 6.3 x 106
o-Xylene 2.4 x 107
m-Xylene 3.6 x 107
p-Xylene 4.3 x 107
Durene 5.5 x 107
Anisole 8.4 x 107
Mesitylene 2.4 x 108
Compound kCT, (Mxsec)-1
Charge transfer must play a role, since the more electron rich molecules are the best quenchers
13.28
β-Cleavage reactions: another example of bond cleavage
OO
CH2•
O•
O
+hν
O
O
O
O
O
OCH3
O
O
CH3O
τ ~0.16 ns
Φ ~ 0.04
τ ~ 50 ns
Φ ~ 0.02
τ ~ 0.1 ns
Φ ~ 0.01
τ ~ 1 ns
Φ ~ 0
Why is the quantum yield so low?
13.29
β Phenyl rings lead to the efficient
intramolecular deactivation of triplet carbonyls
O
O
OCH3
CH3O
OCH3
CH3O
>2 µs 1-3 ns
>10 µs 100-300 ns
An intramolecular example of charge transfer quenching
13.30
β-Cleavage reactions: another example of bond cleavageHiding charge-transfer
O
O
CH2•
O•
O
+hν
O
O
O
O
O
OCH3
O
O
CH3O
τ ~0.16 ns
Φ ~ 0.04
τ ~ 50 ns
Φ ~ 0.02
τ ~ 0.1 ns
Φ ~ 0.01
τ ~ 1 ns
Φ ~ 0
Why are the quantum yields so low?
Because lifetimes are controlled by intramolecular charge transfer deactivation and the nature of the excited state
13.31
Carbonyl triplet addition to electron poor olefinsSome general characteristics
• Only S1 (n,π*) forms oxetanes with cyanoethylenes
• The cis-trans isomerization is a side reaction which is not affected by quenching of the S1 (n,π*) state.
• Quenching of the T1 (n,π*) state sensitizesreactions of cyanoethylene, but it does not lead to oxetane (although the reaction does take place with electron rich olefins).
• Oxetane formation from S1 (n,π*) is stereospecific.
• Oxetane formation from S1 (n,π*) is regiospecific.
Fig 11.7
O*R1
R1
+R2
R2 OR2
R2
R1
R1
OR2
R2
R1
R1
13.32
Addition of acetone to an electron-rich alkene
• Occurs from both triplet and singlet states, but the former is less stereospecific.
• Proposed to involve a biradical
• Charge transfer from the alkene to the ketone is important
O
H3C
H3C
+
H
C2H5H
H3CO
O
H3C
H3C
+
C2H5
HH
H3CO
C2H5
HH
H3CO
H
C2H5H
H3CO
O CH3
CH3
OCH3C2H5
H H
O CH3
CH3
OCH3H
C2H5 H
O CH3
CH3
C2H5H
H3CO H
O CH3
CH3
HH
H3CO C2H5
13.33
Representative examples for n,π* systems
• Atom abstractions
• Radical-like addition
• Electron transfer
• Bond cleavage
Norrish Type II reaction, photoenolizations and intermolecular photoreductions.
Electron rich and electron poor olefins
Amine quenching, exciplex-type mechanisms, β-aryl quenching, aromatic quenching
Norrish Type I reaction and β-cleavage processes
Ground statereactants
Excited statereactants
ReactionIntermediates
Ground stateproducts